Module
ActionDispatch::Routing::UrlFor

In config/routes.rb you define URL-to-controller mappings, but
the reverse is also possible: a URL can be generated from one of your
routing definitions. URL generation functionality is centralized in this
module.

Tip: If you need to generate URLs from your models or some
other place, then ActionController::UrlFor is
what you're looking for. Read on for an introduction. In general, this
module should not be included on its own, as it is usually included by
url_helpers (as in Rails.application.routes.url_helpers).

URL generation from parameters

As you may know, some functions, such as ActionController::Base#url_for and
ActionView::Helpers::UrlHelper#link_to,
can generate URLs given a set of parameters. For example, you've
probably had the chance to write code like this in one of your views:

link_to, and all other functions that require URL generation functionality,
actually use ActionController::UrlFor
under the hood. And in particular, they use the
ActionController::UrlFor#url_for method. One can generate the same path as
the above example by using the following code:

Notice the only_path: true part. This is because UrlFor has no information about the website hostname
that your Rails app is serving. So if you want to include the hostname as
well, then you must also pass the :host argument:

By default, all controllers and views have access to a special version of
#url_for, that already knows
what the current hostname is. So if you use #url_for in your controllers or
your views, then you don't need to explicitly pass the
:host argument.

For convenience reasons, mailers provide a shortcut for
ActionController::UrlFor#url_for. So within mailers, you only have to type
url_for instead of 'ActionController::UrlFor#url_for'
in full. However, mailers don't have hostname information, and you
still have to provide the :host argument or set the default
host that will be used in all mailers using the configuration option
config.action_mailer.default_url_options. For more information
on #url_for in mailers read the
ActionMailer#Base documentation.

URL generation for named routes

UrlFor also allows one to access methods that
have been auto-generated from named routes. For example, suppose that you
have a 'users' resource in your config/routes.rb:

resources :users

This generates, among other things, the method users_path. By
default, this method is accessible from your controllers, views and
mailers. If you need to access this auto-generated method from other places
(such as a model), then you can do that by including Rails.application.routes.url_helpers
in your class:

Missing routes keys may be filled in from the current request's
parameters (e.g. :controller, :action,
:id and any other parameters that are placed in the path).
Given that the current action has been reached through GET
/users/1:

Notice that no :id parameter was provided to the first
url_for call and the helper used the one from the route's
path. Any path parameter implicitly used by url_for can always
be overwritten like shown on the last url_for calls.